Electron bombarded semiconductor devices are receiving more attention for power amplification and control. The electron bombarded semiconductor devices perform beyond some existing devices and offer exciting new possibilities due to their unique capabilities. For example, they lend themselves for broad-band frequency or time domain amplifiers, pulse modulators, harmonic generators, real-time samplers, signal processers and a host of other applications including multi-function performance from a single device.
Simply stated, the relevant theory calls for an electron bombarded semiconductor actuated by an electron beam that effects control by injecting carriers into a back-biased semiconductor diode. A modulated electron beam incident on a reverse-biased semiconductor p-n junction is used to modulate the density of charge carriers and, therefore, the current flowing through the junction. The effect is similar to the modulation produced by a photon beam on a semiconductor; modulation of the photon beam produces, by means of the photoelectric effect, a modulation of the charge carrier density in the semiconductor. Electron beams with beam energies of approximately 10 keV can be used to produce a substantial current gain of the order of more than 10.sup.3 in semiconductor p-n junctions. This is the basis of the hybrid electron beam semiconductor technology. Typical electron beam semiconductor structures consist of an electron-beam source, a high frequency modulation scheme of the electron beam and a p-n junction target all enclosed in a high-vacuum envelope. The modulation is produced by a small potential applied to a grid which controls the electron beam current or a deflection system which moves the electron beam from one diode to another when there are a multitude of diodes arranged in a diode matrix array. So far, the electron bombarded semiconductor device technology has concerned itself primarily with having a silicon p-n junction as the appropriate target.
A most helpful acquaintance with this technology appears in Volume 62, No. 8, August 1974 of the Proceedings of the IEEE in an article entitled "ELECTRON BOMBARDED SEMICONDUCTOR DEVICES" by Aris Silizars, David J. Bates and Aaron Ballonoff. This paper familiarizes a reader with how the devices work and their actual and potential uses. A semiconductor diode is used as a target in a vacuum tube and, when electron beams having an energy between 10 and 15 keV strike the p-n junctions, multiple hole-electron pairs are created by each incident high energy electron. It has been found that a hole-electron pair is created by approximately every 3.6 eV of energy expended, so that each 12 keV incident beam electron can produce thousands of carrier pairs in the diode to result in a current amplification or gain in the neighborhood of 2000 or more. The injection of carriers into a typical diode is accomplished by bombarding the top metal contact with the energized electron beam. Even though the electrons in the beam lose some energy penetrating both the top metal contact layers and a highly doped thin junction region, they enter the depletion region with considerable energy. The energy remaining after penetrating the loss layers is dissipated in the process of forming electron-hole pairs near the junction. One polarity of the carrier moves through the depletion region of the semiconductor causing the current to flow in the load circuit while the other polarity carrier is swept back to the bombarded contact to provide current continuity in the diode circuit.
An electron bombarded semiconductor amplifier employing a GaAs Schottky barrier instead of a p-n junction has been discovered to have power gain up to 1 GHz. This gain includes 50 watts peak pulse power at 150 MHz and 5 watts at 1 GHz, a power gain of 1.3 dB at 1 GHz and efficiency of 17 percent using beam energies of 8 to 20 keV. In contrast with the diode target degradation under electron beam bombardment in silicon p-n junction devices, there has been no such degradation in GaAs Schottky barrier diodes. GaAs devices have other advantages over silicon electron bombarded semiconductor devices because the unsaturated electron velocity of GaAs is larger than that of silicon and the electron transient time of GaAs is shorter than that of silicon. In addition, the GaAs materials appear to be less susceptible to electron beam degradation than silicon although the exact reasons for these phenomena are not clearly understood.
On the other hand, electron bombarded semiconductor devices fabricated wth GaAs can not handle the power levels that the silicon devices can largely because the thermal conductivity of GaAs is only about 60 percent that of silicon. Another limitation of using GaAs is that it has a chemical reactivity inherent in the use of metal Schottky barriers on GaAs. The metal-GaAs metallurgical interreactions cause both short term and long term degradation of Schottky barriers and might well be expected to cause similar problems in electron bombarded semiconductor applications. Irrespective of the limitations of the GaAs semiconductor material, it has been repeatedly demonstrated that, overall, the current gain and frequency response of the GaAs Schottky barriers in electron bombarded semiconductor applications are superior to those of silicon p-n junction. The lower gain of p-n junctions are attributed to recombination losses in the p-layers. Current gains are approximately 1500 for silicon, GaAs and GaAs.sub.x P.sub.1-x Schottky barriers and it was confirmed that GaAs is less susceptible to deterioration than silicon and there were no specific advantages of using GaAs.sub.x P.sub.1-x for electron bombarded semiconductor purposes.
Thus, there is a continuing need in the state of the art for an electron bombarded semiconductor device capable of being excited by electrons which does not damage the semiconductor material, has the capability for large relative current gains and which lends itself to microwave, digital and analog signal processing requirements and for wideband travelling wave tube applications.